1. Field of the Invention
The present invention relates to an Automatic Repeat Request (ARQ) system, and more particularly, to a rate-matching algorithm of a Wideband Code Division Multiple Access (WCDMA) Hybrid ARQ system. Although the present invention is suitable for a wide scope of applications, it is particularly suitable for Hybrid ARQ type II systems with full retransmission.
2. Discussion of the Related Art
Generally, the automatic repeat request (ARQ) and forward error correction (FEC) techniques are well-known error control techniques commonly employed in many communication systems.
In a communication system that employs the ARQ technique, a receiving system makes requests to re-transmit data if the data were received erroneously. Therefore, an object of ARQ is to increase the reliability of the packet data transmission by re-transmitting the same data to the receiving system until the receiving system receives the data error-free. ARQ techniques provide very reliable communication, however they often introduce undesirable delays, decreasing the data throughput rate of the system.
On the other hand, the FEC technique fixes such delay errors by using an error correction code. Therefore, an object of a Hybrid ARQ system, in which both ARQ and FEC techniques are combined, is to have the delay error correction capability of the FEC technique and the low error rates of the standard ARQ technique.
The coding rate of the channel code in an adaptive hybrid ARQ system starts from a source-coding rate, and it decreases as the number of retransmission increases in order to increase reliability of the data transmission. Such hybrid ARQ technique being adaptive to channel environments is essential to a system that provides a multimedia service.
The adaptive ARQ technique is based on an adaptive error correction technique: the number of the surplus data bits gradually increases during the retransmission process. In addition, the adaptive ARQ technique uses one of a “stop and wait”, “go back N”, and “selective repeat” protocol method, and a data packet having a length L includes n information bits and m tail bits.
A typical hybrid ARQ system starts to transmit the data encoded with a reasonably high coding rate, and a mobile station requests to resend the data (sends NAK) if they were received erroneously. Then, the transmitting system, which receives NAK, sends an incremental code word encoded with a lower coding rate.
The hybrid ARQ system often uses any one of convolutional or turbo encoding techniques for encoding a code, and the hybrid ARQ system can be classified into a type II or type III system based on whether the encoded code is a Rate Compatible Punctured (RCP) code or a Complementary Punctured (CP) code. In addition, the system is classified into type I system for re-transmitting the same signal.
There are four different types of hybrid ARQ system: (1) Hybrid ARQ type I system without code combining, (2) Hybrid AIRQ type I system with code combining, (3) Hybrid ARQ type II system with full retransmission, and (4) Hybrid ARQ type II system with partial retransmission.
(I) Hybrid ARQ Type I System Without Code Combining
In this system, a transmitter adds a CRC code to a data packet and performs an error correction coding process before sending the packet. Then, a receiver performs a CRC inspection and requests to retransmit if any error exists. Thereafter, the transmitter resends a same packet, and the receiver decodes the retransmitted packet independently. This system is set as “working assumption” in current 3GPP standards.(2) Hybrid ARQ Type I System With Code CombiningThis system is similar to the Hybrid ARQ type I system shown above except that the receiver initially combines the originally transmitted packet with the retransmitted packet, and it decodes the combined packet. It requires an additional memory space, and the number of data bits representing a symbol for combining must be considered.(3) Hybrid ARQ type II System With Full RetransmissionIn this system, the sender transmits an incremental redundancy after sending a data packet.(4) Hybrid ARQ Type II System With Partial RetransmissionThis system is identical to the Hybrid ARQ type II system with full retransmission except that the re-transmitted data is only partially retransmitted and the size of the incremental redundancy is smaller than the size of the original data pocket.
The current Third Generation Partner Project (3GPP) meetings for WCDMA standards are only trying to decide what type of ARQ system should be used. Therefore, it is crucially important to provide the specific algorithms for hybrid ARQ systems. In addition, in order to achieve the optimal result, various techniques and modification should be applied to such algorithms.